1. Field of the Invention
This invention relates to an improved method of producing acrylonitrile-base in-line dyed fibers, i.e. dyed in the course of the production process. More particularly, the invention concerns a method of producing mainly acrylic or modacrylic fibers which have been wet-spun or dyed in a gel state, that is, a condition occurring during the process step intervening between coagulum and drying.
Such a technology utilizes in particular, as regards gel dyeing, the characteristic of wet-spun acrylic fiber of having a high specific surface area (80-100 m.sup.2 /g) microporous fibrillar structure, and hence a high capacity and rate of absorption. In such conditions, the presence of acid groups imparts the fiber with the property of quickly fixing the basic dyestuffs employed during the dyeing step.
2. Prior Art
The method of producing in-line dyed fibers has been long known. Also known is to dye acrylic fibers in the gel state, e.g. with the method procedures disclosed in the UK Pat. No. 986,114, in Polish Pat. No. 44274, and U.S. Pat. Nos. 3,111,357; 3,242,243; and 3,113,827, as well as in Japanese Pat. No. 12801/65, U.S. Pat. No. 3,907,498 and French Pat. No. 1.389.015.
However, the application of such methods on an industrial scale would involve considerable difficulties and problems, mainly from the gel dyeing step, which difficulties become the greater as the base tow denier rating increases, i.e. the denier rating of the substantially web-like configuration assumed by the fiber bundle after the coagulum step at the successive deflection and dragging members, and as the count of each individual filament in the tow being treated decreases.
One of the major difficulties is concerned with the micro-uniform dye-taking which the dyed tow should exhibit, i.e. the uniform dyestuff spreading which the fibers are to show within a cross-section area through the tow, which property markedly affects the qualities of the finished articles formed from such fibers.
In order to obtain tows having an adequately micro-uniform dyestuff distribution, especially where high denier tows are being processed--on the order of one million denier above, as demanded by today's markets--the prior art proposes production methods which are hardly satisfactory from the plant layout and economical standpoints, and which not always afford results in keeping with the market requirements. In particular, a first solution for dyeing wet-spun fiber in a gel state, as proposed by the prior art, consists of holding the tow as spread out as possible in the dyeing bath, so as to adequately dye each tow filament. However, this procedure involves the availability of complex equipment, extremely sophisticated to operate, owing to the sensitivity of fibers still in the gel state, to spread the tow and then draw it narrower, as well as a more than negligible risk of damaging the fiber while processing it.
Another prior approach consists of lengthening considerably the two residence time in the bath, either by passing the tow through very long dyeing baths having very high bath volumes, or by using very long dyeing time periods while slowing the tow rate of pass through the dyeing bath.
Both such prior methods pose, accordingly, technological problems, and fail to ensure the highly uniform dye-taking feature which is required of in-line dyed finished fiber.
It has presently been unexpectedly found that by passing, through a tow of mainly acrylic or modacrylic wet-spun fibers still in the gel state, some given dyeing bath flow rates in well defined conditions, it becomes possible to alleviate the problems and difficulties mentioned above, while achieving a dyed fiber with highly micro-uniform dye-taking properties.